Allowance for Loan Losses - Age Analysis of Past Due Loans Segregated by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	$ 3,847,627	$ 3,531,605
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	6,448	8,535
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	2,214	3,605
Financing Receivable, Recorded Investment, Non-performing	12,457	22,454
Financing Receivable, Recorded Investment, Past Due Loans, Total	21,119	34,594
Total Portfolio Loans	3,868,746	3,566,199
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	1,674,930	1,595,590
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	2,548	1,209
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	323	207
Financing Receivable, Recorded Investment, Non-performing	4,435	10,750
Financing Receivable, Recorded Investment, Past Due Loans, Total	7,306	12,166
Total Portfolio Loans	1,682,236	1,607,756
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	991,136	836,276
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	1,227	2,599
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	153	278
Financing Receivable, Recorded Investment, Non-performing	1,622	3,296
Financing Receivable, Recorded Investment, Past Due Loans, Total	3,002	6,173
Total Portfolio Loans	994,138	842,449
Commercial Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	214,174	139,133
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	0	1,049
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	0	751
Financing Receivable, Recorded Investment, Non-performing	1,974	2,742
Financing Receivable, Recorded Investment, Past Due Loans, Total	1,974	4,542
Total Portfolio Loans	216,148	143,675
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	485,465	481,260
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	565	828
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	1,220	1,666
Financing Receivable, Recorded Investment, Non-performing	2,336	3,338
Financing Receivable, Recorded Investment, Past Due Loans, Total	4,121	5,832
Total Portfolio Loans	489,586	487,092
Home Equity [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	414,303	408,777
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	1,756	2,468
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	445	659
Financing Receivable, Recorded Investment, Non-performing	2,059	2,291
Financing Receivable, Recorded Investment, Past Due Loans, Total	4,260	5,418
Total Portfolio Loans	418,563	414,195
Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	65,111	67,420
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	352	382
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	73	44
Financing Receivable, Recorded Investment, Non-performing	31	37
Financing Receivable, Recorded Investment, Past Due Loans, Total	456	463
Total Portfolio Loans	65,567	67,883
Consumer Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Current	2,508	3,149
Financing Receivable, Recorded Investment, 30 to 59 Days Past Due	0	0
Financing Receivable, Recorded Investment, 60 to 89 Days Past Due	0	0
Financing Receivable, Recorded Investment, Non-performing	0	0
Financing Receivable, Recorded Investment, Past Due Loans, Total	0	0
Total Portfolio Loans	$ 2,508	$ 3,149

Financing receivable recorded investment non performing.

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- Definition

Financing receivables that are less than 60 days past due but more than 29 days past due.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

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Financing receivables that are less than 90 days past due but more than 59 days past due.

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Financing receivables that are current.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524

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